The previous networks for communicating packet data, although useful in certain situations, have suffered from certain deficiencies. Among these is the inability to efficiently transmit both the packet data and non-packet data such as isochronous data, over the same physical media. Because isochronous-source data is not packetized, it cannot be accommodated in a packet format without substantially interfering with its isochronous character, often introducing an undesirable amount of delay or jitter.
The relationship between packetized data and isochronous data is best discussed after distinguishing between isochronous-source data and isochronous data transfer. In general terms, isochronous data is data which is non-packetized and of indeterminate, potentially continuous duration. An isochronous data source is a device which outputs data in a continuous stream, usually at a substantially constant average data rate. Examples include video cameras, which output a substantially continuous stream of data representing images and associated sounds, and telephone output, which can be a substantially continuous output of voice data (either analog or digitized). An example of an isochronous data sink is a video monitor which can receive a substantially continuous stream of video data for display.
Related to isochronous sources and sinks is the concept of data transfer which can also be either isochronous or non-isochronous. One type of non-isochronous data transfer is a packet-type transfer. As shown in FIG. 1A, data can be transferred in a plurality of packets 12a, 12b which can be either constant-sized or variable-sized. Each packet includes a field of data 14a, 14b which may be preceded and/or followed by non-data information such as preamble information 16a, 16b housekeeping information such as data source information, data destination information, and the like 18a, 18b, in some packet schemes, a frame end marker 20a is provided. In a typical packet system, there is no signal on the medium during the periods between packet transmissions which are of varying, indeterminant length. As seen in FIG. 1A, because the fields provided for data 14a, 14b are not substantially continuous, the packetized scheme of FIG. 1A is not isochronous but is "bursty" in nature. The timing of packet transmissions is irregular, and generally determined by data needs. An example of packetized data transfer is the commonly-used Ethernet system, one implementation of which, known as 10BASE-T is described in the draft Nine supplement to IEEE standard 802.3, dated Nov. 15, 1989.
Another type of non-isochronous data transfer is a token ring system. In a token ring system, a node is permitted to transmit data only after receipt of an electronic "token." As depicted in FIG. 1B, a first station may transmit a token 22a which is received 24a by a second station whereupon the second station may begin transmission of data 26a. After a period of data transmission, the second station transmits the token 22b which is received by a third station 24b that can then begin its own transmission of data 26b. As seen in FIG. 1B, because data transmission is synchronized with the occurrence of an event (the arrival of a token), the token ring system is not an isochronous data transfer system. One commonly used token ring Network is described in IEEE standard 802.5.
In contrast, FIG. 1C schematically depicts isochronous data transfer. In isochronous data transfer, the data transfer or "connection" is initiated, such as by initiating a telephone conversation or beginning a video camera transmission 30. After the connection is initiated, transmission of the data, possibly accompanied by transmission of housekeeping information (such as destinations, audio or video timing, and the like) is provided substantially continuously for an indeterminate period, such as until termination of the connection. Although it may be that not every bit transferred represents a data bit (since "housekeeping" bits may be also transferred), the transfer of data is substantially continuous in the sense that there are no substantial periods during which no data bits are transferred. It is possible that the data being transferred is "Null" data such as silence during a telephone conversation or transfer of a blank video image. One type of isochronous data transfer is the Fiber Distributed Data Interface-II (FDDI-II) as described, for example, in FDDI-II Hybrid Multiplexer, Revision 2.4, dated Mar. 25, 1991.
Accordingly, it would be advantageous to provide an isochronous data communication system in which packetized data, such as Ethernet data, can be transferred across the physical media in a non-packetized frame structure. Preferably, the frame structure can accommodate both the packet-source data and non-packet-sourced data such as isochronous data. Preferably, the system would be substantially backwards-compatible for example with an existing Ethernet system so that existing nodes, e.g. non-isochronous nodes, could be put on the isochronous network and would be able to operate normally. The non-isochronous nodes could later be upgraded to take advantage of isochronous capability, as desired.